I've had either house brand 5W-30 synthetic (turned out to be Caltex C-3), or currently house brand 10W-30 synthetic blend for that last 600-1000 miles. If I had a litre of what's in the sump lying around, it would certainly go in.

Good question, the only recent time I was down a quart was with my 99 Accord 4cyl with 135k miles. I topped off with some Mobil 5000 and was quickly down a quart. Next I topped off with some MaxLife and never had to top it off again. Sold it with 166k miles.

For what its worth, at this point in time i have ST 530 in my 5.4 and thats as cheap as cheap gets... but my next change might be something synthetic, and if it were, id rather not use expensive oil for something that might be changed in a few hundred miles.

I like to use what I've put in at oil change time or something close to it.For example, I'll top up the Maxlife in the BMW with Durablend, since I have some clearance Durablend 10W-40 in handy quarts.Most of our cars don't use enough oil to require adds, so I don't worry about what to add in most cases.I would advise you not to top off obsessively.The engine is perfectly safe down to the add line on the dipstick, which is typically one quart.If you're a quart down at your planned OCI, you're golden.

For the second time, I put Delvac 15W/40 in last oil change, and started topping of with CPC 40Wt. That's marginally cheaper than the Delvac, but my reason for using it was

(a) I had some, and I was out of Delvac(b) It would compensate for any shear down of the VI in the Delvac, though at the very low miles I do in the car that's unlikely to be an issue. (Might be more relevant in the motorcycle.)

Moot point anyway since it initially used hardly any oil, but then it developed a massive oil leak after I removed the sump. I assumed this was the sump gasket (it was the rocker cover gasket) so it took ne a while to fix it, and the sump is now about 60 Delvac: 40 CPC 40Wt.

(Actually it'll be less, cos some of what I put in will have leaked out, but I've added about 40% of the sump volume.)

Perhaps not ideal, but its nearly summer. Can't say I notice any difference in the way it runs.

I top off with what's already opened. My Buick doesn't use any oil whatsoever currently but my Taurus drinks it. My last top off for this run of Mobil 1 High Mileage 5w30 was a gallon jug of PYB api SM that I got on clearance at O'Reilly. I think it's drank 3 quarts in 7k miles.

I wouldn't go out of my way to top off with the same thing, since it's all miscible and new additives are better than depleted additives. My next top off is going to be clearance M1 TDT 5w40, $2 each at AZ, after that on sale peak 5w30 I got at big lots cheap, $1.75 quart iirc.